It's true the aunt passed away at ninety-nine (9)
I believe the answer is:
authentic
'it's true' is the definition.
(thesaurus)
'the aunt passed away at ninety-nine' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'away' could be 'a' (abbreviation as used in sports fixtures) and 'a' is located in the answer.
'ninety nine' could be 'ic' (I've seen this in other clues) and 'ic' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for authentic that I've seen before include "Real, genuine" , "reliable" , "Ian, the cut is genuine" , "Genuine, not a copy" , "Bona fide" .)